The long range goal of this research program is to characterize in molecular terms the mechanism of threonine inhibition of the aspartokinase-homoserine dehydrogenase-I enzyme complex. Special emphasis has been placed upon clarifying the mechanisms which account for the complex interactions between these two linked enzymic activities. Most recently we have synthesized 3-OH pipecolic acid (3-OH PIP), a cyclic threonine analogue which shows a differentially stronger inhibition of the dehydrogenase as compared to the kinase activity. We hope to utilize 3-OH-PIP to characterize the allosteric transition in the enzyme--its preferential binding to one class of threonine binding sites permits substantial simplification of data needed to quantify the energetics of the conformation changes occurring upon inhibitor binding. A key long term goal of this research is to attempt to define critical alterations in the active site(s) of the enzyme which account for the allosteric inhibition of the enzyme by threonine. We hope to further exploit the usage of Co(III) exchange-inert substrate and inhibitor complexes to probe the intersubunit interactions responsible for cooperative inhibition and activation of the enzyme.